Before leaving horizontal continuity we should

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to the spread of surface fires, since fire intensity can be much less in this fuel level. Before leaving horizontal continuity, we should consider other effects of a closed versus open timber canopy. Figure 9 on page 11 illustrates that a forest canopy not only shades surface fuels and prolongs moisture retention but also greatly reduces wind speeds from levels above the canopy to levels near the surface. Generally, the greater the crown closure, the greater the wind speed reduction. This certainly does have an effect on surface fires burning in these closed environments. If torching out of individual trees occurs, however, we have an entirely new fire environment with which to be concerned. We've discussed some aspects of surface fires versus torching out and crown fires. A very important fuels characteristic involved here is the vertical arrangement of fuels. We define vertical arrangement as the relative heights of fuels about the ground as well as their vertical continuity, both of which influence fire reaching various fuel levels or strata.
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In some mature timber situations, we need to be concerned with several levels of fuels which may help transport fire from the surface to the crowns. Figure 10 suggests four levels. Surface fuels mostly consist of grass and litter of various sizes. Low fuels may consist of shrubs, low limbs, and small young trees called regeneration. A subcanopy might consist of understory trees and larger regeneration. The canopy is made up of mature tree crowns perhaps over 100 feet tall. Fire may burn through one or more levels without burning the canopy. Regardless of the maximum height of the fuels and the number of fuel levels involved, we are concerned with the vertical continuity. When fuels are mostly vertically continuous, we call this a fuel ladder, or a ladder to transport fire into the forest canopy. On page 12, please do question 6: Mark your choice or choices, then return to the text. All of the statements given in question 6 are true. As implied in number 4, surface fire intensity is an important factor in whether torching out may actually occur through ladder fuel situations. A very important fuels characteristic is fuel moisture content. It can vary in different fuel levels and thus influence whether those levels become involved with fire. We define fuel moisture content as the amount of water in fuels, expressed as a percent of the oven-dry weight of that fuel. In nature, dead-fuel moisture very seldom gets below 3 or 4 percent. Dead fuel moisture fluctuates considerably over time due to several environmental factors, as shown in figure 11. Live fuel moistures run much higher, perhaps 300 percent or more, but they change less rapidly than dead fuels. This is an interesting area of study that we will resume in Unit 5 of this course, which is entitled "Fuel Moisture." See question 7 on page 13. Mark your choice or choices, then return to the text.
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  • Spring '04
  • MIchealJenkins
  • Combustion, fuel, Wildfire

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